1. Field of the Invention
This invention relates to a tracking control device and tracking control method, which cause a main beam and a sub-beam to converge on an information recording medium having a plurality of information layers, and perform control so as to cause the convergence point of the main beam to scan a track of an information layer, as well as an optical disc device which records or reproduces information in an information recording medium having a plurality of information layers.
2. Description of the Background Art
In recent years, optical disc devices have been widely used for the recording and reproduction of image signals and audio signals.
In an optical disc device, processing to read information written to an information layer of an optical disc is performed by using an optical pickup to scan a fine track with a minute light beam spot converging on the optical information layer. In this case, in order to accurately and continuously read information written to the optical disc, servo technology to cause the light beam spot to follow the track is indispensable.
Hence in optical disc devices, generally tracking control is performed which causes the light beam spot to follow a track based on a tracking error signal (hereafter called a “TE signal”) which detects shifts in the position of the light beam spot from the track center.
Further, in optical disc devices, generally a so-called push-pull method is used as a detection method for detection of the TE signal.
However, in the case of a TE signal detected by the push-pull method, lens shift in which for example an objective lens is shifted in a direction perpendicular to the optical axis of the light beam may occur, or, when there is inclination of an information layer, return light from the light beam reflected from the information layer may be received at a position shifted from the center of the light-receiving face of a detector which is a photoreceiving element. As a result, the TE signal may not be 0 even in a state in which the light beam spot is positioned in the center of the track, so that an offset occurs.
Hence if such a TE signal is used to perform tracking control, the light beam spot does not correctly scan the track center, and a decline in the recording and reproduction performance of the optical disc device may result.
The differential push-pull method is a detection method to prevent offset occurring due to such TE signals. In the differential push-pull method, the light beam irradiating the optical disc is divided into three beams, which are one main beam and two sub-beams. In the differential push-pull method, a main push-pull tracking error signal (hereafter called the “MTE signal”) detected by the main beam, and a sub-push-pull tracking error signal (hereafter called the “STE signal”) detected by the two sub-beams, are generated; by calculating the difference between the MTE signal and a signal obtained by multiplying the STE signal by a coefficient, a differential push-pull tracking error signal (hereafter called the “DPP signal”) is detected.
In the differential push-pull method, offsets occur in both the MTE signal and the STE signal due to lens shifts and information layer inclination. Hence by appropriately setting the coefficient to be multiplied with the STE signal, the offset included in the DPP signal which is a differential signal can be cancelled (see for example Japanese Patent Application Laid-open No. 61-94246).
Further, even when the optical disc reflectivity varies or the light beam irradiation power during recording or reproduction varies, the detection gain for the above-described MIE signal and STE signal is generally held constant by an AGC (Automatic Gain Control) circuit, which normalizes the MTE signal and STE signal by the light quantity of the return light. As a result, stabilized tracking control can be realized.
In an optical disc device in which a main beam and a sub-beam are used to perform tracking control, in general, the intensity ratio of the main beam to the sub-beam is often set to approximately 10:1. In an optical disc device with such a setting, when information is recorded onto or reproduced from a two-layer optical disc or another optical disc having a plurality of information layers, the following problem occurs.
For example, in a two-layer optical disc, when one information layer which is the layer to be accessed is irradiated with a light beam in order to reproduce a signal, light reflected from the one information layer contains light reflected from the other information layer which is not to be accessed (other-layer stray light). It is difficult to separate reflected light from the one information layer and reflected light from the other information layer. For this reason, the other-layer stray light signal component is also detected by a detector. In this case, the other-layer stray light of the main beam is detected by a sub-detector which receives the sub-beam, and the return light quantity of the sub-beam cannot be correctly detected. As a result, an AGC circuit cannot correctly normalize a signal to correct changes in the optical disc reflectivity or changes in the light beam irradiation power during recording and reproduction, and a DPP signal having an appropriate detection gain cannot be obtained. Hence instability in tracking control results, and the recording and reproduction performance of the optical disc device tend to decline.
In order to resolve such problems, an optical pickup has been proposed in which a detector is provided to detect other-layer stray light of the main beam, and by using the result of subtraction of the detected light quantity of other-layer stray light from the sub-beam detected light quantity in the AGC circuit, a DPP signal is generated such that correction of the light quantity of sub-beam return light is performed, and the effect of other-layer stray light is reduced (see for example Japanese Patent Application Laid-open No. 2005-346882).
However, in an optical pickup of the prior art, correction of an other-layer stray light signal component requires a dedicated detector to detect other-layer stray light. Hence the addition of optical components, peripheral circuitry for optical components, and adjustment processes is entailed, and as a result, the problem of increased cost of the optical disc device occurs.